Compatibility agent for aqueous polypigmentary suspensions containing hydrated calcium sulfate

ABSTRACT

Copolymers having the general formula: ##STR1## are useful as compatibility, dispersion and grinding agents for aqueous pigmentary suspensions formulated from a mixture of mineral pigments, at least one of which is hydrated calcium sulfate. These suspensions are useful in papermaking.

This is a division of application Ser. No. 07/418,869, filed on Oct. 10,1989, now U.S. Pat. No. 5,109,090.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compatibility, dispersion and/or grinding agent for aqueous pigmentary suspensions formulated from a mixture of mineral pigments, at least one of which is hydrated calcium sulfate. The invention also relates to aqueous pigmentary suspensions containing the dispersion and/or grinding agent formulated from a mixture of pigments, at least one of which is hydrated calcium sulfate, a method for preparing such suspensions, a method of papermaking, using the above-identified aqueous pigmentary suspensions, and the paper made by such a process.

Background of the invention

When aqueous pigmentary suspensions are prepared from a mixture of mineral pigments, there frequently occurs an extremely rapid and considerable increase in their viscosity, which can reach the solidification point of said suspensions.

This phenomenon, which is linked to the incompatibility of the pigments with one another, occurs for example in suspensions containing a mixture of CaCO₃ and titanium oxide, CaCO₃ and kaolin, and CaCO₃ and titanium oxide and kaolin.

Known to the skilled artisan by the term "shock effect", this viscosifying effect also occurs, but to a greater extent, in the case of aqueous pigmentary suspensions containing both hydrated calcium sulfate and calcium carbonate, or even hydrated calcium sulfate, kaolin and calcium carbonate.

Hydrated calcium sulfate is a mineral pigment which has not yet been greatly exploited as such, but possesses interesting qualities in the field of mass charge or coating of the paper with a view to improving the opacity, shine, reaction to water, printing and whiteness thereof (Patents FR 2,370,124; FR 2,343,082; FR 2,440,437; FI 67.568).

In addition, for the satisfactory use of aqueous pigmentary suspensions formulated from a mixture of mineral pigments, at least one of which is hydrated calcium sulfate, it is desirable to be able to use an agent which is capable of both promoting the dispersion of the pigmentary particles and of eliminating the shock effect by rendering the pigments compatible with one another.

For many years the specialized literature has discussed the research carried out to eliminate the shock effect occurring between the hydrated calcium sulfate and other pigments when they are used together in aqueous pigmentary suspensions.

A first reference (EP 0 216 516) describes a compatibility agent which enables the preparation of aqueous suspensions containing as pigments a mixture of gypsum (in which 70% of the particles have a size of less than 2 microns), kaolin (in which 80% of the particles have a size of less than 2 microns) and calcium carbonate (in which 86% of the particles have a size of less than 2 microns). The recommended compatibility agent is formed from an alkylphosphate ester or a polyacrylate, in which at least 15% of the carboxylic sites are esterified.

In this manner, aqueous suspensions containing a mixture of hydrated calcium sulfate and calcium carbonate or kaolin were prepared in the presence of the above-identified compatibility agent, whether it be an alkylphosphate ester or a partially esterified polyacrylate. However, in order to achieve reasonable viscosities for industrial use, the concentration of these suspensions was unable to go beyond 56% by weight of dry material.

In addition and in the case of the use of an alkylphosphate ester as the compatibility agent, there appears a new restriction of having to operate in a medium with a pH of at most equal to 7 in order for the rheology of the pigmentary suspension to remain within acceptable values for the applications planned. Moreover, when the pH exceeds 7, it is no longer possible to control the viscosity of the mixture of pigments in suspension, with this viscosity increasing very rapidly in a detrimental manner.

Therefore, as the aqueous pigmentary suspensions for the coating of the paper must normally have a concentration of dry material of at least 65% and a pH of between 8 and 11, the compatibility agent recommended in said patent is unusable in the field of paper coating.

In EP 0 227 623, a compatibility agent is proposed which is selected from the acid alkylamine salts in order to regulate the viscosity of aqueous pigmentary suspensions intended for paper coating and containing a mixture of calcium sulfate, kaolin and/or calcium carbonate. However, it has not been possible to bring the content of dry material of these suspensions to a value greater than 56% by weight without causing an increase in viscosity which is prohibitive for paper coating applications.

FR 2,370,124 discloses the use of a compatibility agent to stabilize aqueous pigmentary suspensions intended for paper coating and containing a mixture of aluminum trihydroxide and dihydrated calcium sulfate with a grain size of less than 20 micrometers, with said pigment compatibility agent being a phosphate-ester of depolymerized starch, possibly in combination with a conventional dispersion agent.

However, such pigmentary compositions can be used in the paper coating field only if the user is satisfied with a low content of dry material (on the order of 50 by weight) in order to control the rheology, which fact is certainly a disadvantage.

GB 2,173,781 discloses, for the manufacture of a hydrated calcium sulfate slurry, to use as a dispersing agent a mixture of a surfactant agent with a high concentration, selected from the group of organic sulfates or sulfonates, and a complexing agent selected from the inorganic phosphates and the amines. However, as the present applicants have found, such a mixture of surfactant and complexing agents does not enable the compatibility sought between the hydrated calcium sulfate and the other pigments normally used for paper coating to be achieved.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to provide a compatibility agent which eliminates the previously mentioned shock effect and renders possible the simultaneous presence of hydrated calcium sulfate and other pigments in aqueous pigmentary suspensions without causing the increase in viscosity which is normally observed.

Another object of the present invention is to provide a compatibility agent which eliminates the shock effect and which is also an effective grinding-agent for the refinement of coarse hydrated calcium sulfate, alone or in a mixture with other mineral materials.

Another object of the present invention is to provide aqueous pigmentary suspensions which are more concentrated than in the prior art and which are prepared from mixtures of mineral pigments at least one of which is hydrated calcium sulfate.

Another object of the present invention is to provide a method for preparing aqueous pigmentary suspensions which contain hydrated calcium sulfate.

Another object of the present invention is to provide a method of papermaking using aqueous pigmentary suspensions containing hydrated calcium sulfate.

Another object of the present invention is to provide paper made by a process using aqueous pigmentary suspensions containing hydrated calcium sulfate.

These and other objects, which will become apparent during the course of the following detailed description, have been achieved by the inventors' discovery that copolymers having the general formula (I): ##STR2## symbolized by the formula:

    --A--.sub.K --B--.sub.L --C--.sub.M

wherein (B) is an ethylene monomer having a phosphate or phosphonate function, (A) is an ethylene monomer having a carboxylic acid group, and (C) is an ethylene monomer of the ester or substituted type, and wherein L is a value such that (B) is present in an amount of between 5 and 95.parts by weight, based on the sum of the amounts of (A), (B), and (C), and one of K and M may be 0, are effective compatibility, dispersion, and grinding agents for pigmentary suspensions which contain hydrated calcium sulfate and which may be used in papermaking.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Thus, while the previous reports basically describe compatibility or dispersion agents resulting from mixtures of known products, the agent in accordance with the present invention is distinguished by the fact that it concentrates, due to a single molecular structure, all the functions peculiar to creating interpigmentary effects of compatibility and permitting the dispersion and the grinding of pigments, at least one of which is calcium sulfate.

In addition, whereas the prior art reveals only the placing in suspension of a mixture of pigments containing coarse hydrated calcium sulfate at concentrations not exceeding 56% by weight of dry material, the agent in accordance with the present invention gives rise to a synergistic effect simultaneously providing the refinement and the acquisition of the interpigmentary compatibility effect at concentrations greater than those of the prior art, while favorably controlling the rheology of the suspensions for the papermaking industry.

In the above-given formula of the agent in accordance with the invention, R₁ can be hydrogen or a carboxylic acid group, which may or may not be esterified with a C₁ to C₁₂, preferably C₁ to C₃, alcohol.

R₂ can be hydrogen or a C₁ to C₁₂, preferably C₁ to C₃, alkyl group.

R₃ and R'₃ can be hydrogen, a C₁ to C₁₂, preferably C₁ to C₃, alkyl group, a substituted or an unsubstituted aryl, a carboxylic acid group, or a halogen.

R₄ can be hydrogen, a C₁ to C₁₂, preferably C₁ to C₃, alkyl group, a substituted or an unsubstituted aryl, or a carboxylic acid group which may or may not be esterified with a C₁ to C₁₂, preferably C₁ to C₃, alcohol.

R₅ is a group of atoms binding the phosphorus to the polymeric chain by a phosphate function ##STR3## or by a phosphonate function ##STR4##

Where the function is of the phosphate type, R₅ can be:

either an ethylene oxide or a propylene oxide carboxylic ester of the formula: ##STR5##

in which R₁₀ can be hydrogen or a methyl group and n can take on values in the range of 1 to 50, preferably 1 to 10;

an alkyl, aryl or aralkyl carboxylic ester of the formula: ##STR6## in which R₁₁ can contain 1 to 12 carbon atoms;

a substituted amide of the formula: ##STR7## in which R₁₂ is hydrogen, C₁ to C₁₂, preferably C₁ to C₃, alkyl, a substituted or unsubstituted C₅ or C₆ aryl group, or an aralkyl group, and R₁₃ can be C₁ to C₁₂, preferably C₁ to C₃, alkyl, substituted or unsubstituted C₅ or C₆ cycloalkyl, such as cyclohexyl and cyclopentyl, C₅ or C₆ cycloalkenyl, such as cyclopentadienyl, aryl, such as phenyl or aralkyl; or

oxygen.

In the case where the function is of the phosphonate type, R₅ can be C₁ to C₁₂, preferably Cl to cycloalkyl, substituted or unsubstituted C₅ to C₆ cycloalkenyl, substituted or unsubstituted aryl, aralkyl, a C₁ to C₁₂, preferably a C₁ to C₃, ester, or a substituted amide.

R₆ can be hydrogen, a cation, an ammonium group, an amine, C₁ to C₃ alkyl, substituted or unsubstituted C₅ or C₆ cycloalkyl, substituted or unsubstituted C₅ to C₆ cycloalkenyl, substituted or unsubstituted aryl, aralkyl, a C₁ to C₁₂, preferably C₁ to C₃, ester, or a substituted amide.

R₇ can be hydrogen, a carboxylic group, a C₁ to C₁₂, preferably C₁ to C₃, ester, C₁ to C₁₂, preferably C₁ to C₃, alkyl, C₅ or C₆ cycloalkyl, C₅ or C₆ cycloalkenyl, aryl, or aralkyl.

R₈ can be hydrogen, a carboxylic group, C₁ to C₃ alkyl, or a halogen.

R₉ can be a C₁ to C₁₂, preferably C₁ to C₅, ester, a substituted or an unsubstituted amide, C₁ to C₁₂, preferably C₁ to C₃, alkyl, C₅ or C₆ cycloalkyl, C₅ or C₆ cycloalkenyl, aryl, aralkyl, a halogen, a carboxylic group, or a sulfonated or sulfated alkyl or aryl group.

These acid copolymers may be prepared by copolymerization in accordance with conventional methods, in the presence of appropriate initiators and regulators, in an aqueous, alcoholic, hydroalcoholic, aromatic or aliphatic medium or in a halogenated solvent, of at least two ethylenically unsaturated monomers one of which has a phosphate or phosphonate function, at atmospheric or over pressure. That is, the present copolymers may be prepared by the copolymerization of an ethylenically unsaturated monomer corresponding to unit (B) in formula (I) with at least one ethylenically unsaturated monomer corresponding to either unit (A) or (C) in formula (I).

Preferred monomers corresponding to unit (A) include acrylic acid, methacrylic acid, itaconic acid, and maleic acid. Preferred monomers corresponding to unit (C) include esters of acrylic and methacrylic acid, such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, and butyl methacrylate.

Thus, the polymerization medium can be water, methanol, ethanol, propanol, isopropanol, butanols, or even dimethylformamide, dimethylsulfoxide, tetrahydrofuran, acetone, methylethylketone, ethyl acetate, butyl acetate, hexane, heptane, benzene, toluene, ethylbenzene, xylene, mercaptoethanol, tert-iododecylmercaptan, thioglycolic acid and esters thereof, n-dodecylmercaptan, acetic acid, tartaric acid, lactic acid, citric acid, gluconic acid, glucoheptonic acid, 2-mercaptopropionic acid, thiodiethanol, halogenated solvents such as carbon tetrachloride, chloroform, methylene chloride, methyl chloride, monopropyleneglycol ether or diethyleneglycol ether.

The copolymers intended to be used as a compatibility, dispersion and grinding agent in accordance with the present invention are generally selected from among those having a specific viscosity of between 0.2 and 10.0 and preferably between 0.25 and 5.0, and most preferably between 0.4 and 1.2.

The specific viscosity of the copolymers, which is symbolized by the letter "η" was determined in the following manner:

A copolymer solution in sodium salt form was prepared by dissolving 50 g of dry copolymer in one liter of a solution of distilled water containing 60 g of sodium chloride.

Then, using a capillary viscometer placed in a 25° C. thermostatted bath, the flow time of a given volume of the above-identified solution containing the alkaline copolymer was measured, as was the flow time of the same volume of aqueous solution of sodium chloride without said copolymer. It was then possible to define the specific viscosity "η" using the following equation: ##EQU1##

The capillary tube is generally selected such that the flow time of the NaC₁ solution without the copolymer is approximately 90 to 100 seconds, thus giving very accurate measurements of specific viscosity.

At the end of the polymerization, the acid copolymers in accordance with the invention in the aqueous solution are collected and can be used in this form.

In accordance with one alternative, which has been shown to be very interesting, the copolymers in accordance with the invention in the aqueous solution can be totally or partially neutralized by a neutralizing agent having a monovalent function. In addition, a neutralizing agent having a polyvalent function can be combined with this monovalent agent.

In the first case, the neutralizing agent is desirably selected from the group consisting of alkaline and similar cations, particularly lithium, sodium, potassium, ammonium or amine polysubstituted or not. This agent, in the second case, can be combined with an agent from the alkaline earth or similar group, preferably calcium and magnesium.

In practice, the liquid phase resulting from the copolymerization and containing the acid copolymer can be used in a salified form as a slurrying or grinding agent, but it can also be dried by any known means in order to remove said liquid and isolate the copolymer in the form of a fine powder and can be used in this other form as a compatibility, dispersion or grinding agent.

In another embodiment, the agent in accordance with the invention can be used in accordance with a method for the preparation of aqueous pigmentary suspensions, containing at least hydrated calcium sulfate, comprising the following steps:

(a) The preparation by slurrying of an aqueous suspension of hydrated calcium sulfate, alone or mixed, is carried out under stirring by first placing all or part of the agent in accordance with the invention into the aqueous phase, and then introducing the pigment(s) so as to obtain a fluid and desirably homogeneous suspension;

(b) The suspension prepared in accordance with step a) is continuously introduced into a grinding zone composed of a grinder with microelements;

(c) The temperature of the suspension during grinding is kept at a level below 100° C. and preferably between room temperature and 80° C;

(d) The suspension is ground in the presence of the microelements for the time necessary to obtain the desired particle size;

(e) During the grinding, an additional fraction of the agent is possibly introduced, in at least one step;

(f) On exiting the grinder, the finely ground suspension of CaSO₄ ·2H₂ O, alone or in a mixture, is continuously separated from the grinding elements and the particles which are rejected as being too coarse; and

(g) At least one rheology-modifying agent is possibly introduced into the finely ground suspension.

If the pigment or pigments used have a grain size distribution which conforms to the requirements of the user, only step (a) is carried out. In the contrary case, the following steps are carried out, that is (b) to (g), so as to produce an aqueous suspension of pigments refined by grinding and rendered compatible with each other.

The agent in accordance with the invention is introduced into the aqueous pigmentary suspension in an amount of 0.05 to 4% by weight of active material in relation to the dry weight of the mineral materials, and preferably in an amount of 0.5 to 2.5%.

The suspension collected after the slurrying and/or the grinding generally has a concentration of at least 65% dry material, and preferably in the range of 70 to 80% by weight, with the size of the pigmentary particles obtained being defined by the user in the case of grinding.

The pigmentary suspension refined in this manner, due to the presence of the agent in accordance with the invention, can be preferably used as a pigmentary charge in the manufacture of paper, paint and plastic materials.

Other features of the invention will become apparent in the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

EXAMPLES Example 1

This example, the object of which is to illustrate the effects of the known agents, relates to the preparation of a suspension of hydrated calcium sulfate using slurrying.

For this purpose, several suspensions of hydrated calcium sulfate were prepared with 19 types of known dispersion or compatibility agents.

All the suspensions (Tests 1 to 19) were carried out with the same calcium sulfate having an average grain size on the order of 15 micrometers, with the concentration of the suspensions being fixed at 68% by weight of dry material and the amount of the agent adjusted to 1% by weight of active material in relation to the pigmentary material.

The suspensions were prepared in the same apparatus as in step a) of the previously described method at room temperature.

The properties and results of Tests 1 to 19 are given in Table 1.

                                      TABLE 1                                      __________________________________________________________________________     Slurrying of CaSO.sub.4.2H.sub.2 O with Known Agents                                                                               BROOKFIELD                                             CONCENTRATION                                                                             AVERAGE DIAMETER                                                                            VISCOSITY                                     PERCENTAGE                                                                              OF THE     OF PARTICLES IN                                                                             after 24 hrs               NUMBER                                                                               TYPE OF POLYMER                                                                             OF AGENT SUSPENSION MICROMETER   10 REVS                                                                             100                   __________________________________________________________________________                                                              REVS                   1    Sodium polyacrylate -                                                                       1        68         15           2000  650                        Specific viscosity 0.40                                                   2    Sodium polyacrylate -                                                                       1        68         15           1800  700                        Specific viscosity 0.57                                                   3    Sodium polyacrylate -                                                                       1        68         15           2700  770                        Specific viscosity 0.30                                                   4    Sodium polyacrylate -                                                                       1        68         15           3700 1300                        Specific viscosity 0.70                                                   5    Sodium polymethacrylate -                                                                   1        68         15           18000                                                                               3900                        Specific viscosity 0.55                                                   6    Chromium lignosulfonate                                                                     1        68         15           10200                                                                               1600                   7    C.sub.6 -Alcohol phosphate                                                                  1        68         15           2100  800                        monoester acid                                                            8    C.sub.10 -Alcohol phosphate                                                                 1        68         15           4100 1450                        monoester acid                                                            9    C.sub.6 -Alcohol phosphate                                                                  1        68         15           SET IN MASS                      monoester sodium salt                                                    10    C.sub.10 -Alcohol phosphate                                                                 1        68         15           SET IN MASS                      sodium salt                                                              11    Calcium dodecyl                                                                             0.6      68         15           8400 1680                        benzenesulfonate +                                                                          0.4                                                               Sodium pyrophosphate                                                     12    Sodium C.sub.16 -olefinsul-                                                                 0.6      68         15           7200  950                        fonate + Sodium                                                                             0.4                                                               pyrophosphate                                                            13    Carboxy methyl                                                                              0.6      68         15           8100 1120                        cellulose +  0.4                                                               C.sub.6 -Alcohol monoester                                                     phosphate acid                                                           14    Carboxy methyl                                                                              0.6      68         15           10200                                                                               1700                        cellulose +  0.4                                                               C.sub.10 -Alcohol monoester                                                    phosphate acid                                                           15    Ethylene glycol                                                                             0.6      68         15           7800 1080                        methacrylate monoester                                                                      0.4                                                               phosphate + Carboxy                                                            methyl cellulose                                                         16    Oxyethyl 18-tristyryl-                                                                      1        68         15           5300  920                        phenol phosphate                                                         17    2-methylpropanesulfonic                                                                     1        68         15           9500 1540                        acrylamide copolymer (30)                                                      Acrylic acid (37)                                                              Methacrylic acid (33)                                                          Specific viscosity: 0.4                                                  18    Itaconic acid copolymer                                                                     1        68         15           10800                                                                               2700                        (15) Methacrylic acid (40)                                                     Acrylic acid (45)                                                              Specific viscosity: 0.4                                                  19    Phosphonic salt (Dequest                                                                    1        68         15           8000  950                        160 from Monsanto)                                                             0.4 + Copolymer No. 18                                                   __________________________________________________________________________

Table 1 demonstrates that the viscosities at 10 and 100 revolutions are fairly high for a monopigment, in certain cases resulting in setting.

EXAMPLE 2

This example, which is intended to illustrate the object of the invention, relates to the slurrying of hydrated calcium sulfate (of the same origin as in Example 1) in the presence of the agent in accordance with the invention in the case where the copolymer does not contain monomer (C) (that is M=0).

Through Tests 20 to 29, the formula of the copolymer was modified by qualitatively and quantitatively varying the various radicals as well as the specific viscosities of the copolymers used, it being understand that said copolymers had been at least partially salified with sodium cations until a pH of between 7 and 8 on average was obtained

The properties of the suspensions prepared (Tests 20 to 29) and the results obtained are collected in Table 2.

    TABLE 2       Slurrying of CaSO.sub.4.2H.sub.2 O According to the Present Invention           Average Brookfield       Concentration Diameter Viscosity       of      the of the after 24 hrs Example A B C Specific Percentage suspension      Particles (cP) No R1 R2 M1 k R3 R4 R5 R6 M2 l R7 R8 R9 m Vicosity of      Agent (%) (micrometers) 10 rev 100 rev                             20 H  H Na 90  H CH.sub.3       ##STR8##       Na Na/H 10  0 0,65 1   68 15 800 520  21 H  H Na 85  H CH.sub.3       ##STR9##       Na Na/H 15  0 0,58 1   68 15 200 120       22 H  H Na 85  H H      ##STR10##       Na Na/H 15  0 0,74 1   68 15 800 440  23 H  H Na 70  H CH.sub.3       ##STR11##       Na Na/H 30  0 0,76 0,3 68 15 850 600  24 H  H Na 70  H CH.sub.3       ##STR12##       Na Na/H 30  0 0,5  0,4 68 15 1050  750  25 H  H Na 60  H CH.sub.3       ##STR13##       Na Na/H 40  0 0,56 0,1 68 15 950 485  26 H  H Na 50  H CH.sub.3       ##STR14##       Na Na/H 50  0 0,43 0,1 68 15 800 400  27 H  CH.sub. 3 Na 60  H CH.sub.3       ##STR15##       Na Na/H 40  0 0,41 0,1 68 15 850 420  28 H  HCH.sub.3 NaNa 4515  H      CH.sub.3       ##STR16##       Na Na/H 40  0 0,51 0,1 68 15 750 360  29 H  H Na 20  H CH.sub.3       ##STR17##       Na Na/H 40  0 insoluble 0,1 68 15 720 320               H H       ##STR18##       Na Na/H 40

From Table 2 and the comparison with Table 1, it is apparent that the viscosities of the suspensions in accordance with the invention measured under the same conditions as those of Example 1 are considerably lower than obtainable by the known agents. Therefore, it was possible to lower the concentration of compatibility agent by large amounts, possibly by a ratio of 10/1.

Example 3

This example, which is intended to illustrate the object of the invention, relates to the slurrying of hydrated calcium sulfate (of the same origin as in Example 1) in the presence of the agent in accordance with the invention, in the case where the copolymer does not contain monomer (A) (that is K=0).

Through Tests 30 to 35, the formula of the copolymer was modified by qualitatively and quantitatively varying the various radicals as well as the specific viscosities of the copolymers used, it being understand that said copolymers had been a least partially salified with the sodium cation until a pH of between 7 and 8 on average was obtained.

The properties of the suspensions prepared (Tests 30 to 35) and the results obtained are collected in Table 3.

    TABLE 3       Slurrying of CaSO.sub.4.2H.sub.2 O According to the Present Invention           Average Brookfield       Concentration Diameter Viscosity       of      the of the after 24 hrs Example A B C Specific Percentage suspension      Particles (cP) No R1 R2 M1 k R3 R4 R5 R6 M2 l R7 R8 R9 m Vicosity of      Agent (%) (micrometers) 10 rev 100 rev                             30  0 H CH.sub.3       ##STR19##       Na Na/H 40 H  H       ##STR20##       60   1   68 15 650 430        31  0 H CH.sub.3      ##STR21##       Na Na/H 30 H  HCH.sub.3 CO(CH.sub.2).sub.2 OHOCH.sub.3 4525   1   68 15      500 310       32  0 H CH.sub.3 -- Na Na/H 40 H  H      ##STR22##       60  0,6  0,5 68 15 1040  420  33  0 H CH.sub.3 -- Na Na/H 40 H      CH.sub.3       ##STR23##       60  0,55 0,5 68 15 950 370  34  0 H CH.sub.3 -- Na Na/H 40 H  HCH.sub.3       ##STR24##       4515  0,7  0,5 68 15 1000  405       35  0 H H      ##STR25##       Na Na/H 30 H  H       ##STR26##       70  0,65 1        68 15 900 560

From Table 3 and the comparison with Table 1, it is apparent that the viscosities of the suspensions in accordance with the present invention, measured under the same conditions as those of Example 1, are considerably lower than obtainable by the known agents Therefore, it was possible to lower the concentration of compatibility agent by large amounts, possibly by a ratio of 2/1.

Example 4

This example, which is intended to illustrate the object of the invention, relates to the slurrying of hydrated calcium sulfate (of the same origin as in Example 1) in the presence of the agent in accordance with the invention, in the case where the copolymer prepared contains the three monomers (A), (B) and (C), that is when K and M are simultaneously not equal to 0.

Through Tests 36 to 60, the formula of the copolymer was modified by qualitatively and quantitatively varying the various radicals as well as the specific viscosities of the copolymers used, it being understand that said copolymers had been at least partially salified with the sodium cation until a pH of between 7 and 8 on average was obtained.

The properties of the suspensions prepared (Tests 36 to 60) and the results obtained are collected in Table 4.

    TABLE 4       Slurrying of CaSO.sub.4.2H.sub.2 O According to the Present Invention         Per-  Average Brookfield Ex-     cen- Concentra- Diameter Viscosity      am-     tage tion of the of the after 24 hrs ple A B C Specific of      suspension Particles (cP) No R1 R2 M1 k R3 R4 R5 R6 M2 l R7 R8 R9 m      Vicosity Agent (%) (micrometers) 10 rev 100 rev                             36 H H Na 70 H CH.sub.3       ##STR27##       Na Na/H 15 H CH.sub.3       ##STR28##       15 0,46 0,5 68 15 910 630       37 H H Na 65 H CH.sub.3      ##STR29##       Na Na/H 20 H CH.sub.3       ##STR30##       15 0,5  0,5 68 15 840 560       38 H H Na 55 H CH.sub.3      ##STR31##       Na Na/H 30 H CH.sub.3       ##STR32##       15 0,66 0,3 68 15 680 340       39 H H Na 40 H CH.sub.3      ##STR33##       Na Na/H 30 H CH.sub.3       ##STR34##       30 0,6  0,3 68 15 600 320       40 H H Na 45 H CH.sub.3      ##STR35##       Na Na/H 40 H CH.sub.3       ##STR36##       15 0,54 0,1 68 15 710 450       41 H H Na 55 H H      ##STR37##       Na Na/H 15 H CH.sub.3       ##STR38##       30 0,55 0,5 68 15 1020  730       42 H H Na 30 H H      ##STR39##       Na Na/H 40 H CH.sub.3       ##STR40##       30 0,53 0,3 68 15 700 440       43 H CH.sub.3 Na 45 H CH.sub.3      ##STR41##       Na Na/H 40 H CH.sub.3       ##STR42##       15 0,49 0,2 68 15 680 470       44 H H Na 45 H CH.sub.3      ##STR43##       Na Na/H 40 H H       ##STR44##       15 0,60 0,1 68 15 780 510       45 H H Na 45 H CH.sub.3      ##STR45##       Na Na/H 40 H H       ##STR46##       15 0,55 0,1 68 15 820 550       46 H H Na 55 H CH.sub.3      ##STR47##       Na Na/H 30 H H CO(CH.sub.2).sub.2CH.sub.3 15 0,52 0,2 68 15 920 590  47      H H Na 50 H CH.sub.3       ##STR48##       Na Na/H 40 H H CO(CH.sub.2 CH.sub.2 O).sub.4 (CH.sub.2).sub.11CH.sub.3      10 0,50 0,4 68 15 840 530 48 H H Na 45 H CH.sub.3       ##STR49##       Na Na/H 40 H H CN 15 0,56 0,2 68 15 780 470  49 H H Na 45 H CH.sub.3       ##STR50##       Na Na/H 40 H CH.sub.2 COOH COOH 15 0,55 0,3 68 15 720 510  50 H H Na 45      H CH.sub.3       ##STR51##       Na Na/H 40 H CH.sub.3       ##STR52##       15 0,48 0,2 68 15 690 450       51 H H Na 45 H CH.sub.3      ##STR53##       Na Na/H 40 H H C.sub.6 H.sub.5 15 0,7  0,1 68 15 850 540  52 H H Na 45      H CH.sub.3 -- Na Na/H 40 H CH.sub.3       ##STR54##       15 0,65 0,1 68 15 750 520       53 H H Na 45 H CH.sub.3      ##STR55##       Na Na/H 40 H CH.sub.3       ##STR56##       15 0,7  0,4 68 15 910 670  54 H H Na 45 H H CH.sub.2 O Na Na/H 40 H      CH.sub.3       ##STR57##       15 0,8  0,4 68 15 860 590  55 H H Na 45 H C.sub.6 H.sub.5 -- Na Na/H 40      H CH.sub.3       ##STR58##       15 Insoluble 0,3 68 15 780 460       56 H H Na 45 H H      ##STR59##       Na Na/H 40 H CH.sub.3       ##STR60##       15 0,68 0,4 68 15 990 600       57 H H Na 55      ##STR61##       H       ##STR62##       Na Na/H 30 H CH.sub.3       ##STR63##       15 0,50 0,4 68 15 850 540       58      ##STR64##       H Na 45 H CH.sub.3       ##STR65##       Na Na/H 40 H CH.sub.3       ##STR66##       15 0,75 0,3 68 15 930 625       59 H H Na 45 H CH.sub.3      ##STR67##       Na Na/H 40 H H       ##STR68##       15 0,5  0,3 68 15 720 440       60 H H Na 45 H CH.sub.3      ##STR69##       Na Na/H 40       ##STR70##       H       ##STR71##       15 0,58 0,6 68 15 970 640

From Table 4 and the comparison with Table 1, it is apparent that the viscosities of the suspensions in accordance with the present invention, measured under the same conditions as those of Example 1, are considerably lower than obtainable by the known agents. Therefore, it was possible to lower the concentration of compatibility agent by large amounts, possibly by a ratio of 10/1.

Example 5

This example, which is intended to illustrate the object of the present invention, relates to the slurrying of hydrated calcium sulfate (of the same origin as in Example 1) in the presence of the agent in accordance with the invention, copolymer with the formula described in Example 4 and with a specific viscosity varying from 0.27 to 2.5 and salified with the sodium cation.

In this case, several tests were carried out (Tests 40/01 to 40/10), with the aqueous suspensions of hydrated calcium sulfate subjected to slurrying in the presence of said agent in an amount of 0.1% by weight of dry material in relation to the weight of the dry pigment, with the concentration of the suspension always being fixed at 68% by weight of said sulfate.

The properties of the suspensions prepared (Tests 40,,01 to 40/10) and the results obtained are collected in Table 5.

                  TABLE 5                                                          ______________________________________                                         Influence of the Specific Viscosity of the Agent on the                        Rheology of the Pigmentary Suspension                                          Test  Specific   Brookfield Viscosity after 24 hours (in cP)                   No.   Viscosity  10 rpm        100 rpm                                         ______________________________________                                         40/01 0.27       1330          910                                             40/02 0.45       850           520                                             40/03 0.5        840           485                                             40/04 0.54       710           450                                             40/05 0.6        700           410                                             40/06 0.7        650           370                                             40/07 0.76       630           340                                             40/08 0.9        750           460                                             40/09 1.17       900           540                                             40/10 2.5        1250          830                                             ______________________________________                                    

From Table 5 and the comparison with Table 1, it is apparent that the viscosities of the suspensions in accordance with the present invention, measured under the same conditions as those of Example 1, are considerably lower than obtainable by the known agents, even within the claimed limits of specific viscosities. Therefore, it was possible in all cases to lower the concentration of compatibility agent by a ratio of 10/1.

Example 6

This example, which is intended to illustrate the object of the present invention, relates to the slurrying of the same hydrated calcium sulfate used in the preceding examples in the presence of the agent composed of a copolymer with the same specific viscosity and with the same composition but salified with sodium, lithium, ammonium, potassium, calcium and magnesium, or in an acid form, or salified by an amine.

In this case, several tests were carried out (Tests 61 to 69), with the aqueous suspensions subjected to slurrying in the presence of said agent having concentrations of dry material which were fixed at 68% by weight and the amount of the agent being fixed at 0.1% by weight of dry material.

The properties of the suspensions prepared (Tests 61 to 69) and the results obtained are collected in Table 6.

    TABLE 6       Slurrying of CaSO.sub.4.2H.sub.2 O According to the Present Invention           Average Brookfield       Concentration Diameter Viscosity       of      the of the after 24 hrs Example A B C Specific Percentage suspension      Particles (cP) No R1 R2 M1 k R3 R4 R5 R6 M2 l R7 R8 R9 m Vicosity of      Agent (%) (micrometers) 10 rev 100 rev                             61 H H Na 45 H CH.sub.3       ##STR72##       Na Na/H 40 H CH.sub.3       ##STR73##       15 0,54 0,1 68 15  710 450        62 H H NH.sub.4 45 H CH.sub.3      ##STR74##       NH.sub.4 NH.sub.4       /H 40 H CH.sub.3      ##STR75##       15 0,54 0,1 68 15  820 490       63 H H K 45 H CH.sub.3      ##STR76##       K K/H 40 H CH.sub.3       ##STR77##       15 0,54 0,1 68 15 1200 790       64 H H Li 45 H CH.sub.3      ##STR78##       Li Li/H 40 H CH.sub.3       ##STR79##       15 0,54 0,1 68 15  650 400       65 H H Ca 45 H CH.sub.3      ##STR80##       Ca Ca/H 40 H CH.sub.3       ##STR81##       15 0,54 0,1 68 15 1800 950       66 H H NaMg 45 H CH.sub.3      ##STR82##       Na/Mg Na/Mg/H 40 H CH.sub.3       ##STR83##       15 0,54 0,1 68 15 1600 920       67 H H TEA 45 H CH.sub.3      ##STR84##       TEA TEA/H 40 H CH.sub.3       ##STR85##       15 0,54 0,1 68 15 1560 880       68 H H H 45 H CH.sub.3      ##STR86##       H H 40 H CH.sub.3       ##STR87##       15 0,54 0,1 68 15 1100 720       69 H H Na 45 H CH.sub.3      ##STR88##       Na Na 40 H CH.sub.3       ##STR89##       15 0,54 0,1 68 15       700 430

From Table 6 and the comparison with Table 1, it is apparent that the viscosities of the suspensions in accordance with the present invention are considerably lower than obtainable by the known agents, even though the concentration of the compatibility agent was decreased by a ratio of 10/1.

Example 7

The object of this example is to compare the compatibility of aqueous polypigmentary suspensions containing two pigments, at least one of which is hydrated calcium sulfate, for coating paper, using a known compatibility agent (Table 7-2, Tests 70 to 79) and an agent in accordance with the present invention (Table 7-2, Tests 80 to 88).

For this purpose, aqueous monopigmentary suspensions of calcium carbonate, hydrated calcium sulfate, kaolin, and titanium oxide in the presence of a known compatibility agent (types A, B₁, C, and D) and an agent in accordance with the present invention (type B₂) with similar specific viscosities were prepared by slurrying.

The characteristics of the aqueous pigmentary suspensions prepared in this manner and the results obtained are collected in Table VII-1 below, indicating in particular the viscosities of the suspensions at 10 and 100 revolutions per minute.

                                      TABLE 7-1                                    __________________________________________________________________________     Properties of the Suspensions Used in Table 7-2                                                         Suspension                                                                             Average                                                                             Brookfield viscosity                                Agent         Concentration                                                                          diameter                                                                            (cP)                                     Suspension of:                                                                            Type       %  (%)     (microns)                                                                           10 rpm                                                                              100 rpm                             __________________________________________________________________________     CaCO.sub.3 H 90                                                                         A Sodium polyacrylate                                                                       0.4                                                                               75      1    700  300                                 from OMYA Co.                                                                             Specific viscosity: 0.55                                            CaSO.sub.4.2H.sub.2 O                                                                   B1                                                                               Polyacrylate                                                                              1  70      15   2700 980                                            Specific viscosity: 0.6                                                      B2                                                                               Agent of the invention                                                                    1  70      15   200  150                                            Test 40/07                                                          SPS kaolin                                                                              C Sodium polyacrylate                                                                       0.2                                                                               66      3.5  600  300                                 from ECC   Specific viscosity: 0.40                                            AT1 titanium                                                                            D Sodium and calcium                                                                        0.35                                                                              73      0.5  6600 1200                                from Thann polyacrylate                                                        and Mulhouse                                                                              Specific viscosity: 0.4                                             __________________________________________________________________________      The mixtures of the suspensions were prepared by adjusting the amount of       agent to 1%:                                                                   - for the prior art with sodium polyacrylate with a specific viscosity of      0.55                                                                           - for the invention with 40/07.                                          

Then, in order to compare the efficacy of the compatibility agents used, the preparation of aqueous polypigmentary suspensions was carried out by mixing the above-identified suspensions of calcium carbonate, kaolin, and titanium oxide with the hydrated calcium sulfate suspensions.

The properties of the polypigmentary suspensions prepared in this manner and the results obtained from the experiments are provided in Table 7-2.

                                      TABLE 7-2                                    __________________________________________________________________________     Test for Compatibility by Mixing                                                           % by weight of the mixture of suspensions                                                           Brookfield viscosity                                  Dry of  of      of   of  of the mixture                                Test    extract                                                                            CaCO.sub.3                                                                         CaSO.sub.4.2H.sub.2 O                                                                  titanium                                                                            kaolin                                                                             (in cP)                                       number  (%) A   B2  B1  D    C   10 rpm                                                                              100 rpm                                  __________________________________________________________________________     PRIOR ART                                                                      70      74.5                                                                               90      10  0    0   Set in mass                                   71      74  80      20  0    0   "                                             72      73.5                                                                               70      30  0    0   48000                                                                               7200                                     73      66.4                                                                               0       10  0    90  Set in mass                                   74      66.8                                                                               0       20  0    80  "                                             75      67.2                                                                               0       30  0    70  "                                             76      67.6                                                                               0       40  0    60  100000                                                                              10000                                    77      68  0       50  0    50  75000                                                                               10000                                    78      72.7                                                                               0       10  90   0   Set in mass                                   79      72.4                                                                               0       20  80   0   Set in mass                                   INVENTION                                                                      80      66.4                                                                               0   10      0    90  33000                                                                               5200                                     81      66.8                                                                               0   20      0    80  16000                                                                               3500                                     82      67.2                                                                               0   30      0    70   6200                                                                               1300                                     83      74.5                                                                               90  10      0    0   35000                                                                               4000                                     84      74  80  20      0    0   22000                                                                               2840                                     85      73.5                                                                               70  30      0    0   14000                                                                               2000                                     86      72.7                                                                               0   10      90   0    6250                                                                                815                                     87      72.4                                                                               0   20      80   0    2700                                                                                460                                     88      72.1                                                                               0   30      70   0    1600                                                                                295                                     __________________________________________________________________________

Table 7-2 leads to the following observations. The hydrated calcium sulfate, due to its greater solubility in the aqueous medium, is the pigment which generates the shock effect during the preparation of polypigmentary suspensions. However, it should be recalled that for the coating of the paper, it is the polypigmentary suspensions with a low content of hydrated calcium sulfate which are used and which have been shown to be the most sensitive to the shock effect.

For Tests 70 to 79, which represent the effect of known agents to which the invention is compared, the polypigmentary suspensions, obtained by mixing, practically all manifest setting during the mixing of the constituent suspensions. These results reveal the inability of the known compatibility agents to overcome the shock effect caused by the hydrated calcium sulfate.

For Tests 80 to 88, which illustrate the present invention, the polypigmentary suspensions obtained by mixing all have a reasonably decreased viscosity, rendering them suitable to use for the coating of paper. The phenomenon observed of the substantial lowering of the viscosities shows the ability of the present compatibility agent to overcome the shock effect caused by the hydrated calcium sulfate. Therefore, the present compatibility agent enables the preparation of polypigmentary suspensions containing hydrated calcium sulfate with concentrations of dry material which are adequate for papermaking applications.

All the above results were able to be compared by using identical dry extracts, percentages of compatibility agent and identical amounts of pigmentary mixtures for both the known agents and the present agents.

Example 8

The object of this example is to illustrate, in a comparative manner, the ability of the present agent to be the compatibility, the dispersion, and the grinding agent simultaneously.

For this purpose, coarse suspensions of hydrated calcium sulfate from the same origin were prepared by using known compatibility agents (Tests 89 to 92) and compatibility agents according to the present invention (Tests 93 to 98).

Known Compatibility Agents:

Test 89: sodium polyacrylate.

Test 90: sodium polymethacrylate.

Test 91: copolymer of acrylic acid and methallyl sulfonate neutralized with the sodium ion.

Test 92: copolymer of acrylic acid and dimethylaminoethyl methacrylate neutralized with the sodium ion.

Compatibility Agents From the Present Invention

Test 93: copolymer of acrylic acid and ethylene glycol methacrylate phosphate neutralized with the sodium ion.

Test 94: copolymer of methyl methacrylate, ethylene glycol methacrylate phosphate and acrylic acid neutralized with the sodium ion.

Test 95: copolymer of ethylene glycol methacrylate phosphate, ethylene glycol acrylate phosphate and acrylic acid neutralized with the sodium ion.

Test 96: copolymer of ethylene glycol methacrylate phosphate, ethylene glycol acrylate phosphate and methyl methacrylate neutralized with the sodium ion.

Test 97: copolymer of ethylene glycol methacrylate phosphate, acrylic acid and acrylamide neutralized with the sodium ion.

Test 98: copolymer of methyl methacrylate, ethylene glycol methacrylate phosphate and acrylic acid neutralized with the lithium ion.

The average diameter of the hydrated calcium sulfate before grinding was 15 micrometers.

These tests were all carried out using the same experimental procedures, carrying out the grinding in the same apparatus, in order that the results obtained could be directly compared.

For each test, an aqueous suspension of hydrated calcium sulfate with an average diameter of 15 micrometers was prepared.

The aqueous suspension had a concentration of 68% of dry material expressed as a percentage by weight.

The grinding agent was present in the suspensions in an amount of 1% by weight of dry material in relation to the mass of hydrated calcium sulfate to be ground.

Each suspension prepared in this manner was placed in a Dyno-Mill type grinder with a fixed cylinder and a rotating impeller, the grinding body of which was composed of corundum bearings of a diameter in the range of 0.6 millimeter to 1.0 millimeter.

The total volume occupied by the grinding body was 5,000 milliliters, while its weight was 10 kilograms.

The grinding chamber had a volume of 5,000 milliliters. The circumference speed of the grinder was 10 meters per second.

The hydrated calcium sulfate suspension was recycled at a rate of 50 liters per hour.

The exit of the Dyno-Mill grinding was provided with a 300 micron mesh separator, enabling the separation of the suspension resulting from the grinding and the grinding body.

The temperature during each grinding test was maintained at 25° C.

At the end of the grinding, the viscosity of the microparticulate suspension was measured using a Brookfield viscosimeter, at a temperature of 20° C. and at rotation speeds of 10 and 100 revolutions per minute.

Similarly, at the end of the grinding, the grain size was determined using a Cilas Alcatel 715 laser granulometer.

The properties of the ground suspensions and the results obtained after grinding are given in Table 8.

                                      TABLE 8                                      __________________________________________________________________________                                     Grain size                                                                              Brookfield viscosity                  Test                       Specific                                                                            Average diameter                                                                        (in cP)                               number  Composition of the agent                                                                          viscosity                                                                           (in microns)                                                                            10 rpm                                                                              100 rpm                          __________________________________________________________________________     PRIOR ART                                                                      89      100% AA neutralized 100% Na                                                                       0.55 8        17000                                                                               2700                             90      100% AMA neutralized 100% Na                                                                      0.5  9        18000                                                                               3980                             91       85% AA neutralized 100% Na                                                                       0.4  9        12500                                                                               2300                                      15% MTAS                                                              92       50% AA neutralized 100% Na                                                                       0.4  8        16500                                                                               2600                                      50% MADAM                                                             INVENTION                                                                      93       70% AA neutralized 100% Na                                                                       0.76 5         6000                                                                               1000                                      30% PO.sub.4 MAEG                                                     94       15% MAM neutralized 100% Na                                                                      0.76 3         7500                                                                               1120                                      40% PO.sub.4 MAEG                                                              45% AA                                                                95       40% PO.sub.4 MAEG neutralized 100% Na                                                            0.6  4        10000                                                                               1480                                      40% PO.sub.4 AEG                                                               20% AA                                                                96       40% PO.sub.4 MAEG neutralized 100% Na                                                            0.74 5         9500                                                                               1300                                      40% PO.sub.4 AEG                                                               20% MAM                                                               97       40% PO.sub. 4 MAEG neutralized 100% Na                                                           0.62 4         7800                                                                               1100                                      45% AA                                                                         15% Acrylamide                                                        98       15% MAM Neutralized                                                                              0.76 3         7000                                                                                950                                      40 PO.sub.4 MAEG 100% Li                                                       45% AA                                                                __________________________________________________________________________

The results given in Table 8 lead to the following observations. For Tests 89 to 92 which represent suspensions of hydrated calcium sulfate ground in the presence of known compatibility agents, it is seen that the average diameter of the ground pigment remains coarse and that the viscosity of the suspensions obtained is high. Thus, these suspensions are unsuitable for papermaking applications.

For Tests 93 to 98 which represent suspensions of hydrated calcium sulfate ground in the presence of the compatibility agent of the present invention, it is observed that the average diameter of the ground pigment is very fine and that the viscosity of the suspensions obtained is much lower than that obtained by the prior art, even with the consequent decrease in the average diameter.

The favorable development of these two characteristics renders the present suspensions particularly suitable for papermaking applications.

Example 9

The object of this example is to illustrate the capability of the present compatibility agent to enable the grinding of aqueous polypigmentary suspensions containing a pigmentary mixture of hydrated calcium sulfate and calcium carbonate.

For this purpose, mixed suspensions of coarse hydrated calcium sulfate (average diameter of 15 micrometers) and coarse calcium carbonate (average diameter of 30 micrometers) were prepared using a known compatibility agent (Tests 99 to 101) and a compatibility agent in accordance with the present invention (Tests 102 to 104).

Each aqueous suspension had a concentration of 70% by weight of dry material, with the composition of the pigmentary mixture being prepared according to the ratios indicated in Table 9 and the compatibility agent being present in an amount of 1% by weight of dry material in relation to the dry pigments.

The grinding tests were carried out using the same experimental procedures as those described in Example 8.

The properties of the ground suspensions and the results obtained after grinding are given in Table 9.

                                      TABLE 9                                      __________________________________________________________________________     Cogrinding of CaSO.sub.4.2H.sub.2 O and CaCO.sub.3                             Test    Type      Percentage                                                                           Average diameter    Viscosity after grinding           number  of agent  of agent                                                                             after grinding                                                                          CaSO.sub.4.2H.sub.2 O                                                                 CaCO.sub.3                                                                         10 revs.                                                                             100 revs.                    __________________________________________________________________________     PRIOR ART                                                                       99     Sodium    1     GRINDING 70     30  GRINDING                                   polyacrylate    IMPOSSIBLE          IMPOSSIBLE                         100     Specific  1              50     50                                             viscosity: 0.55                                                        101               1              30     70                                     INVENTION                                                                      102     AGENT OF THE                                                                             1     1.3      70     30  14000 1320                         103     INVENTION 1     1.4      50     50  14000 1600                         104     TEST 40-07                                                                               1     1.5      30     70   6000 1160                         __________________________________________________________________________

The results given in Table 9 lead to the following observations: For Tests 99 to 101 which represent the aqueous polypigmentary suspensions using known agents, grinding is impossible due to the rapid increase in viscosity until a paste state is reached.

For Tests 102 to 104 which represent the aqueous polypigmentary suspensions in accordance with the present invention, it can be seen that the average diameter of the two types of ground pigments is very close to one micrometer, revealing not only a remarkable improvement in relation to the pigmentary suspensions of the prior art but also in relation to the ground monopigmentary suspensions of Example 8. In addition, the viscosities after grinding of said suspensions remain reasonable, even low, in spite of the increased fineness of the co-ground pigments.

Example 10

The object of this example is to illustrate the compatibility between pigments which are different in composition when they are used together in an aqueous polypigmentary suspension, due to the presence of the present agent.

For this purpose, monopigmentary suspensions of calcium carbonate and hydrated calcium sulfate were first prepared by grinding in an aqueous medium (using the procedures of Example 8) in the presence of a known compatibility agent (suspension types E and G) and an agent in accordance with the present invention (suspension types F and H). The properties of the monopigmentary suspensions and the results obtained are given in Table 10-1.

                                      TABLE 10-1                                   __________________________________________________________________________     Properties of the Suspensions of Table 10-2                                               Suspension of CaCO.sub.3                                                                             Suspension of CaSO.sub.4.2H.sub.2 O           Reference  E         F           G         H                                   __________________________________________________________________________     Type of agent                                                                             Sodium polyacrylate                                                                      Agent of the invention.                                                                    Sodium polyacrylate                                                                      Agent of the invention.                                  Test 40/07            Test 40/07                          specific viscosity                                                                        0.55      0.76        0.55      0.76                                Concentration of the                                                                      75        75          68        68                                  suspension (%)                                                                 Average diameter                                                                          1         1.5         8         3                                   (microns)                                                                      Brookfield viscosity of                                                                   10 rpm                                                                              100 rpm                                                                             10 rpm                                                                               100 rpm                                                                              10 rpm                                                                              100 rpm                                                                             10 rpm                                                                               100 rpm                       the suspensions (cP)                                                                      700  300  3000  1200  17000                                                                               2700 7500  1170                          __________________________________________________________________________

In order to comparatively test the efficacy of the compatibility agents, the preparation of aqueous polypigmentary suspensions was carried out by mixing suspensions of calcium carbonate and hydrated calcium sulfate, containing on the one hand the known compatibility agent and on the other hand the compatibility agent in accordance with the present invention.

Tests 105 to 108 represent the suspensions with known agents and Tests 109 to 112 represent those of the present agent.

The properties of the polypigmentary suspensions prepared and the results obtained collected in Table 10-2.

                  TABLE 10-2                                                       ______________________________________                                         Dry extract  % by wt. of the                                                                               Brookfield                                         of the       mixture of suspensions                                                                        viscosity                                          Test   mixture   CaCO.sub.3                                                                             CaSO.sub.4.2H.sub.2 O                                                                   (in cP)                                      No.    (%)       E     F   G    H     10 rpm                                                                               100 rpm                            ______________________________________                                         Prior Art                                                                      105    74.3      90    0   10   0     75000 >10000                             106    73.6      80    0   20   0     62000 >10000                             107    72.9      70    0   30   0     45000 >10000                             108    72.2      60    0   40   0     37000   8000                             Invention                                                                      109    74.3       0    90   0   10     3000   740                              110    73.6       0    80   0   20     1600   360                              111    72.9       0    70   0   30     400    230                              112    72.2       0    60   0   40     280    200                              ______________________________________                                    

The results given in Table 10-2 lead to the following observations. For tests 105 to 108, the known compatibility agent does not eliminate the interpigmentary shock effect during the mixing of the monopigmentary suspensions: the viscosities of the known agent-containing polypigmentary suspensions are exacerbated to the extreme with the occurrence of a pasty appearance.

For Tests 109 to 112, the present compatibility agent eliminates the interpigmentary shock effect during the mixing of the monopigmentary suspensions; the viscosities of said polypigmentary suspensions are very low, even lower than those observed in Table 7-2 of Example 7 (polypigmentary suspensions obtained by mixing unground monopigmentary suspensions).

Example 11

The object of this example is to illustrate the capability of hydrated calcium sulfate to be substituted as a pigment for titanium oxide in a polypigmentary suspension, in the coating of paper, due to the presence of the agent in accordance with the present invention.

For this purpose, two aqueous polypigmentary suspensions were prepared: one containing kaolin, calcium carbonate, and titanium oxide dispersed in the aqueous phase by means of a known compatibility agent; the other containing kaolin, calcium carbonate and hydrated calcium sulfate dispersed in the aqueous phase by means of the present compatibility agent.

To these aqueous polypigmentary suspensions, the various components were added which were necessary for their transformation into paper-coating formulas, with the components being introduced in identical amounts so that the two formulas could be distinguished from one another only by the presence of TiO₂ in one, and by the presence of hydrated calcium sulfate and the compatibility agent of the present invention in the other.

These two formulas were used for coating a cardboard on a flat table machine built by the Allimand company.

Using a trailing blade, a coat of 11 to 12 g.m⁻² of each formula was deposited on a cardboard support (170 g.m⁻²) at an application speed of 380 meters per minute in a width of 3.2 meters. At the exit of the Allimand machine, a coated cardboard of 181 to 182 g.m⁻² was obtained.

The properties of the coating formulas and the coated cardboards are given in Table 11.

                  TABLE 11                                                         ______________________________________                                         Coating Formulas and Industrial Tests                                          ______________________________________                                         Kaolin from the                                                                               65           65                                                 ECC Company                                                                    CaCO.sub.3 Hydrocarb 90                                                                       25           25                                                 from OMYA                                                                      TiO.sub.2 RL 68 from Thann                                                                    10            0                                                 and Mulhouse                                                                   CaSO.sub.4.2H.sub.2 O                                                                          0           10                                                 Compatibility agent of the                                                                     0           4% in dry/dry in                                   invention 40/07; addition   relation to the                                                                sulfate                                            CMC Finnfix 5 (water                                                                          0.5% on dry  0.5% on dry                                        retainer) from pigment      pigment                                            Metralliiton Tilisuus                                                          Acrylic styrene latex SD                                                                       15%          15%                                               215 from Rhone Poulenc                                                         Urecoll S (rendering the                                                                      10% in relation to                                                                          10% in relation to                                 layer insoluble) from                                                                         the dry latex                                                                               the dry latex                                      BASF                                                                           NaOH, quantity sufficient                                                                     8.5-8.7      8.5-8.7                                            for a pH of:                                                                   Water, quantity sufficient                                                                     65%          65%                                               for a dry extract of:                                                          Leucophor CK bluing                                                                           1% as is on the                                                                             1% as is on the                                    agent from Sandoz                                                                             dry pigment  dry pigment                                        Brookfield viscosity                                                            10 revs.      7600         6000                                               100 revs.      1280         1000                                               Whiteness        84.5         89.7                                             Brilliance under ultra-                                                                       29           27                                                 violet                                                                         Pause opacity index                                                                            -0.8           -8.64                                           Ink absorption - Drying                                                                       12/15 seconds                                                                               4/5 seconds                                        time                                                                           Behavior on the machine                                                                       Slight clogging                                                                             Perfectly smooth                                                  of the blade blade                                              ______________________________________                                    

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

What is claimed as new and desired to be secured by Letters Patent of the United States is:
 1. A process for making paper, comprising the step:(i) coating a paper support with a suspension comprising hydrated calcium sulfate and a water-soluble copolymer having the formula (I): ##STR90## wherein L is a value such that the amount of the comonomer of the formula ##STR91## is between 5 and 95 parts by weight, based on the total weight of said copolymer, and one of K and M may be 0 and the sum of K, L, and M equals 100 parts by weight; wherein R₁ is one member selected from the group consisting of hydrogen and a carboxylic acid group which may be esterified with a C₁ to C₁₂ alkyl group; R₂ is one member selected from the group consisting of hydrogen and C₁ -C₁₂ alkyl; R₃ and R'₃ are each, independently, one member selected from the group consisting of hydrogen, a carboxylic acid group, halogen, C₁ -C₁₂ alkyl, and substituted or unsubstituted aryl; R₄ is one member selected from the group consisting of hydrogen, C₁ -C₁₂ alkyl, substituted or unsubstituted aryl, and a carboxylic acid group which may be esterified with a C₁ 14 C₁₂ alkyl group; R₅ is one member selected from the group consisting of an ester group, a substituted amide, oxygen, C₁ -C₁₂ alkylene, substituted or unsubstituted arylene, substituted or unsubstituted cycloalkylene, substituted or unsubstituted cycloalkenylene and substituted or unsubstituted aralkylene, an ethylene or propylene oxide carboxylic ester of the formula: ##STR92## in which R₁₀ is hydrogen or methyl and n has a value of between 1 and 50, an alkyl, aryl, or aralkyl carboxylic ester of the formula: ##STR93## wherein R₁₁ contains 1 to 12 carbon atoms, and a substituted amide of the formula: ##STR94## wherein R₁₂ is hydrogen, C_(1-C) ₁₂ alkyl, substituted or unsubstituted aryl, or aralkyl, and R₁₃ is C₁ -C₁₂ alkylene substituted or unsubstituted arylene, substituted or unsubstituted cycloalkylene, substituted or unsubstituted cycloalkenylene, or substituted or unsubstituted aralkylene; R₆ is one member selected from the group consisting of hydrogen, a cation, an amine, C₁ -C₃ alkyl, substituted or unsubstituted aryl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted cycloalkenyl, substituted or unsubstituted aralkyl, a C₁ -C₁₂ ester, and a substituted amide; R₇ is one member selected from the group consisting of hydrogen, a carboxylic acid group, a C₁ -C₁₂ ester, C₁ -C₁₂ alkyl, aryl, cycloalkyl, cycloalkenyl, and aralkyl; R₈ is one member selected from the group consisting of hydrogen, a carboxylic acid group, C₁ -C₃ alkyl, and halogen; and R₉ is one member selected form the group consisting of a C₁ -C₁₂ ester, substituted or unsubstituted amide, C₁ -C₁₂ alkyl, cycloalkyl, cycloalkenyl, aryl, aralkyl, halogen, --CO₂ H, and a sulfonated or sulfated alkyl or aryl group.
 2. Paper, produced by the process, comprising the steps of(i) coating a paper support with a suspension comprising hydrated calcium sulfate and a water-soluble copolymer having the formula (I): ##STR95## wherein L is a value such that the amount of the comonomer of the formula ##STR96## is between 5 and 95 parts by weight, based on the total weight of said copolymer, and one of K and M may be 0 and the sum of K, L, and M equals 100 parts by weight; wherein R₁ is one member selected from the group consisting of hydrogen and a carboxylic acid group which may be esterified with a C₁ to C₁₂ alkyl group; R₂ is one member selected form the group consisting of hydrogen and C₁ -C₁₂ alkyl; R₃ and R'₃ are each, independently, one member selected from the group consisting of hydrogen, a carboxylic acid group, halogen, C₁ -C₁₂ alkyl, and substituted or unsubstituted aryl; R₄ is one member selected for the group consisting of hydrogen, C₁ -C₁₂ alkyl, substituted or unsubstituted aryl, and a carboxylic acid group which may be esterified with a C₁ -C₁₂ alkyl group; R₅ is one member selected from the group consisting of an ester group, a substituted amide, oxygen, C₁ -C₁₂ alkylene, substituted or unsubstituted arylene, substituted or unsubstituted cycloalkylene, substituted or unsubstituted cycloalkenylene and substituted or unsubstituted aralkylene, an ethylene or propylene oxide carboxylic ester of the formula: ##STR97## in which R₁₀ is hydrogen or methyl and n has a value of between 1 and 50, an alkyl, aryl, or aralkyl carboxylic ester of the formula: ##STR98## wherein R₁₁ contains 1 to 12 carbon atoms, and a substituted amide of the formula: ##STR99## wherein R₁₂ is hydrogen, C₁ -C₁₂ alkyl, substituted or unsubstituted aryl, or aralkyl, and R₁₃ is C₁ -C₁₂ alkylene, substituted or unsubstituted arylene, substituted or unsubstituted cycloalkylene, substituted or unsubstituted cycloalkenylene, or substituted or unsubstituted aralkylene; R₆ is one member selected form the group consisting of hydrogen, a cation, an amine, C₁ -C₃ alkyl, substituted or unsubstituted aryl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted cycloalkenyl, substituted or unsubstituted aralkyl, a C₁ -C₁₂ ester, and a substituted amide; R₇ is one member selected from the group consisting of hydrogen, a carboxylic acid group, a C₁ -C₁₂ ester, C₁ -C₁₂ alkyl, aryl, cycloalkyl, cycloalkenyl, and aralkyl; R₈ is one member selected from the group consisting of hydrogen, a carboxylic acid group, C₁ -C₃ alkyl, and halogen; and `R₉ is one member selected from the group consisting of a C₁ -C₁₂ ester, substituted or unsubstituted amide, C₁ -C₁₂ alkyl, cycloalkyl, cycloalkenyl, aryl, aralkyl, halogen, --CO₂ H, and a sulfonated or sulfated alkyl or aryl group. 